This invention relates to sound reproduction systems.
The invention is particularly, but not exclusively, concerned with the stereophonic reproduction of sound whereby signals recorded at a plurality of points in the recording space such, for example, at the notional ear positions of a head, are reproduced in the listening space, by being replayed via three loudspeaker channels, the system being designed with the aim of synthesising at a plurality of points in the listening space an auditory effect obtaining at corresponding points in the recording space.
Binaural technology [1]-[3] is often used to present a virtual acoustic environment to a listener. The principle of this technology is to control the sound field at the listener's ears so that the reproduced sound field coincides with what would be produced when he is in the desired real sound field. One way of achieving this is to use a pair of loudspeakers (electro-acoustic transducers) at different positions in a listening space with the help of signal processing to ensure that appropriate binaural signals are obtained at the listener's ears [4]-[8].
It is also possible to use three channels of loudspeakers for binaural reproduction. It has been experimentally observed by several workers that the addition of another centre channel can improve the cross-talk cancellation achieved with two channel binaural reproduction systems. For example Miyoshi and Koizumi [9] presented a filter design technique for enhanced cross-talk cancellation when three loudspeakers are used in place of two loudspeakers, this method of design following from that previously presented by Miyoshi and Kaneda [10] for the inversion of room acoustic responses. A similar approach that used three loudspeakers was presented by Uto et al [11] who used an adaptive filter design technique. Finally Cooper and Bauck [12] also later disclosed a three channel filter design technique based on the analytical frequency domain inversion of the Moore-Penrose pseudo-inverse matrix of transfer functions relating the loudspeaker outputs to the listener ear signals.
We discuss hereafter in Section 2 a number of problems which arise from these conventional approaches to system inversion involved in such a binaural synthesis over loudspeakers. A basic analysis with a free field transfer function model illustrates the fundamental difficulties which such systems can have. The amplification required by the system inversion results in loss of dynamic range. The inverse filters obtained are likely to contain large errors around ill-conditioned frequencies. Regularisation is often used to design practical filters but this also results in poor control performance. The performance suffers severely even with small errors in the reproduction stage. The Optimal Source Distribution (OSD) provided the solution for all the above problems by introducing the concept of variable frequency span transducers [13].